This invention relates to semiconductor device fabrication methods, and in particular to the formation of optoelectronic and electronic semiconductor devices.
It is very difficult to grow high quality thin film materials on conventional prior art substrates with a large lattice mismatch. Such substrates include but are not limited to Si, GaAs, InP, GaP, GaSb, InAs, and sapphire. In the prior art, either a thick buffer layer has to be grown on the substrate, as shown in U.S. Pat. No. 5,285,086, or a special technique, such as the lateral growth method, as disclosed by Parillaud and et al., Appl. Phys. Lett. 68, 2654, 1996, was employed before the growth of the device structure layers. It is known that defects, in particular threaded dislocations, induced by the lattice mismatch can be reduced from 1011 per square centimeter to 105 per square centimeter by using the lateral growth method. It is difficult to grow reliable epitaxial layers that can be used to fabricate optoelectronic or other electronic devices, due to the high density of the threaded dislocation defects. Prior art substrates exhibit either lattice mismatch or optical absorption that render them unsuitable for use in the growth of optoelectronic or electronic devices.
One such prior art substrate is GaSB. This material is the only material that has served in the prior art as the substrate for type II Sb-based optoelectronic devices, is GaSb. However, commercially available GaSb substrates either have high defect density, greater than 104 per square centimeter, or poor quality in surface smoothness. In contrast, GaAs substrates are characterized by relatively high surface smoothness. The typical FWHM value of a low defect density GaAs substrate is less than 20 arc seconds compared to greater than 100 arc seconds for GaSb substrates. Indeed, GaSb substrates from some vendors have surface roughness that can be observed using optical microscopes with low magnification.
A further disadvantage of using prior art GaSb substrates in optoelectronic devices is the strong absorption of GaSb at wavelengths shorter than 1530 nm. Referring to FIG. 1, there is shown a prior art technique with a sub-mount 112, bonding material 115 on submount 112, GaSb substrate 120 on bonding material 115, and epitaxial layer 125 on substrate 120. Due to the strong absorption of GaSb, a fabricated laser bar has to be attached to the sub-mount using this configuration, known as an epi-up configuration. In the epi-up configuration, heat removal from the active region, i.e., epitaxial layer 125, requires conduction of heat through the GaSb substrate, which has poor thermal conductivity. Even when the substrate has been thinned down to 70 nm, heat removal from the laser active region and across the GaSb substrate is still not efficient. This poor efficiency in removing heat from the active region causes the active region to heat up. Higher temperatures in the active region will increase internal loss and threshold pump power density and reduce the laser output power.
Since, as noted above, high quality GaAs substrate, characterized by a low occurrence of defects, is commercially available and as GaAs does not absorb strongly around the 980 nm wavelength, it would be highly desirable to grow Sb-base type-II optically pumped laser devices on a GaAs substrate. However, the lattice mismatch between AlSb or GaSb, and GaAs is 7.8%. A lattice mismatch such as this between the substrate and the epitaxial thin film is sufficiently great as to induce high defect density, greater than 1011 cmxe2x88x922, in the epitaxial thin film. It is difficult to fabricate reliable and high power mid-infrared (IR) lasers, or any other electronic device, from a high defect-density epitaxial layer.
A high quality compliant universal (CU) substrate is needed to grow antimony based, nitrogen-based, and phosphor (P)-based optoelectronic or electronic devices. In the prior art, Lo proposed, in U.S. Pat. No. 5,294,808, issued Mar. 15, 1994, to use a thin substrate having a thickness on the same order as the critical thickness (the thickness at which defects form when growing one lattice mismatched material on another). A disadvantage of this approach is that the critical thickness is only few hundred angstroms, and it is difficult to sustain the mechanical and chemical processes required for epitaxial growth and device fabrication on a substrate having a thickness of only a few hundred angstroms.
A CU substrate fabricated by twist bonding AlGaAs/GaAs to GaAs has been demonstrated (Z. H. Zhu et al., IEEE J. Selected Topics in Quantum Electronics 3, 927, 1997) to be a candidate for a suitable CU substrate. However, it is very difficult to etch GaAs substrate away from AlGaAs thin film with a good yield. Jet etching has been used as the tool to etch away GaAs in a reasonable time frame. The liquid acid jet pressure applied on the substrate surface enhances the GaAs etch rate but also causes damage in the thin compliant substrate. It is thus more labor intensive and more difficult to manufacture a substrate of this type because it is necessary during the jet etching process to monitor continuously for possible damage to the thin CU substrate.
To address the problem of lattice mismatch between Sb-base thin films used in Mid-IR (MIR) optoelectronic devices and any other III-V heteroepitaxy layer, a CU substrate is needed that can grow high quality epitaxial thin film with lattice mismatch up to 20%. This new CU substrate should be able to grow any high quality III-V thin film and also sustain both sample preparation before and during epitaxial growth and device fabrication after epitaxial growth using molecular beam epitaxy (MBE) or MOCVD.
Additional objects and advantages of the invention will become evident from the detailed description of a preferred embodiment which follows.
According to the present invention, a compliant substrate has a base layer and a thin-film layer thereon and loosely bonded thereto. The thin-film layer has a high degree of lattice flexibility.
According to another aspect of the present invention, a compliant substrate has a base layer and a thin-film layer thereon. The base layer and the thin-film layer have different lattice constants, and are bonded together with or without twisting bonding.